Energy Sinks for Lee Waves in Shear Flow
Energy Sinks for Lee Waves in Shear Flow
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DOI：
10.1175/jpo-d-19-0052.1
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发表时间：
2019-10
影响因子：
3.5
通讯作者：
E. Kunze;R. Lien
中科院分区：
文献类型：
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作者：
E. Kunze;R. Lien
在德雷克海峡和科尔盖伦高原两侧的微结构测量发现，湍流耗散率ε平均比线性背风波产生预测小2-3倍，比基于精细尺度剪切和应变的已建立的参数化预测小3倍。本文探讨了这些差异是波作用守恒的结果的可能性，L=E/|Ku|。波作用守恒将把一部分背风波辐射传输回平均流，如果波遇到减弱的电流U，其中固有频率或拉格朗日频率ωL=|Ku|↓|f|和沿流的水平波数，其中kU≡k⋅V。湍流损失的耗散部分取决于固有频率在产生和破裂之间的多普勒频移，因此取决于地形高度谱和带宽N/f。对于典型的地形高度谱形状和在深海中发现的n/f比，假定阻塞是局部的，对平均流的耗散和损失之间的分配被量化。虽然部分背风波的产生总是在旋转的流体中消散，但背风波并不像先前所说的那样是平衡能量的汇或湍流源。对于南大洋深部典型的地形谱斜率和浮力频率，耗散分数为0.44-0.56，对流速、U和地形分裂不敏感。背风波也是在其产生的底层流中重新分配平衡能量的重要机制。
Microstructure measurements in Drake Passage and on the flanks of Kerguelen Plateau find turbulent dissipation ratesεon average factors of 2–3 smaller than linear lee-wave generation predictions, as well as a factor of 3 smaller than the predictions of a well-established parameterization based on finescale shear and strain. Here, the possibility that these discrepancies are a result of conservation of wave actionE/ωL=E/|kU| is explored. Conservation of wave action will transfer a fraction of the lee-wave radiation back to the mean flow if the waves encounter weakening currentsU, where the intrinsic or Lagrangian frequencyωL= |kU| ↓ |f| andkthe along-stream horizontal wavenumber, wherekU≡k⋅V. The dissipative fraction of power that is lost to turbulence depends on the Doppler shift of the intrinsic frequency between generation and breaking, hence on the topographic height spectrum and bandwidthN/f. The partition between dissipation and loss to the mean flow is quantified for typical topographic height spectral shapes andN/fratios found in the abyssal ocean under the assumption that blocking is local in wavenumber. Although some fraction of lee-wave generation is always dissipated in a rotating fluid, lee waves are not as large a sink for balanced energy or as large a source for turbulence as previously suggested. The dissipative fraction is 0.44–0.56 for topographic spectral slopes and buoyancy frequencies typical of the deep Southern Ocean, insensitive to flow speedUand topographic splitting. Lee waves are also an important mechanism for redistributing balanced energy within their generating bottom current.